In general, a spacecraft, such as an artificial satellite that obtains information while orbiting around the earth, is provided with an attitude control system for controlling an attitude thereof along a track. The attitude control system is configured such that a driving force generated by a reaction wheel or a thruster is exerted on the spacecraft in a desirable direction as needed, and thereby the attitude of the spacecraft is controlled. To accurately and precisely control the attitude of the spacecraft, the driving force should be respectively applied in axial directions of three axes that are perpendicular to each other, such as the x-axis, y-axis and z-axis.
Recently, a spacecraft attitude control system using a sphere, in which the attitude of the spacecraft is controlled by using a single actuator, has been actively researched, wherein a plurality of electromagnets is provided and spaced apart from each other at 90 degrees on the periphery of a sphere that is disposed in the center of the three axes, and a current is periodically applied to the electromagnets such that a rotating magnetic field is generated on the periphery of the sphere, and thereby the driving force is simultaneously exerted on the three axes by Lorentz force exerted on the sphere.
When the spacecraft attitude control system using the sphere is used to control the attitude of the spacecraft, a computer simulation is performed in advance in order to test reliability and control performance of the spacecraft attitude control system. To perform a simulation, another electromagnet is disposed at an upper portion of the attitude control system, and the sphere is levitated by a magnetic field generated by the electromagnet without falling off by the gravity, thereby staying at a predetermined position, which is disclosed in Korean Patent Application publication No. 10-2014-0014634 as “Sphere magnetic levitation system”.
FIG. 1 shows a conventional sphere magnetic levitation system 100.
Referring to FIG. 1, the conventional sphere magnetic levitation system 100 is capable of rotating a sphere 10 in a predetermined direction in a state where the sphere 10 is levitated by using electromagnetic force generated from a plurality of electromagnets 20 disposed on the periphery of the sphere 10. The sphere magnetic levitation system 100 shown in FIG. 1 is configured such that the sphere 10 is levitated by the electromagnetic force, and then rotates without mechanical contact with surrounding components, whereby it is possible to generate high-speed torque with low power consumption.
However, the conventional sphere magnetic levitation system (100) shown in FIG. 1 is problematic for the following reasons.
When the sphere 10 is not levitated by the electromagnets 20, the sphere 10 is not suspended, so when moving the system 100 or changing the direction thereof, the sphere 10 and the electromagnets 20 may be damaged by an impact against the surrounding components caused by vibrations.
When the sphere 10 is levitated by the electromagnets 20, that is, when the sphere 10 rotates at a high speed, an impact between the sphere 10 and the surrounding components may occur by losing the levitation force, causing damage to the entire system 100 when the entire system 100 is out of order or power is cut off due to unforeseen reasons.
To solve the problem described above, a method that mechanically holds the sphere 10 by using a support frame and a ball bearing may be devised. However, in this case, a mechanical friction force may occur because of no magnetic levitation effect, whereby the sphere may not rotate at a high speed, and power consumption may be increased.